1. Field of the Invention
The present invention relates to methods of measuring carbon dioxide elimination. Particularly, the present invention relates to a method of measuring carbon dioxide elimination that compensates for "noise" in respiratory flow measurements, such as leaks in a breathing circuit or system drift, that causes inaccuracies in tidal volume and carbon dioxide elimination calculations and may cause inconsistencies between carbon dioxide elimination calculations.
2. Background of Related Art
Carbon dioxide elimination (VCO.sub.2) is the volume of carbon dioxide (CO.sub.2) excreted from the body of a patient during respiration. Conventionally, carbon dioxide elimination has been employed as an indicator of metabolic activity. Carbon dioxide elimination has also been used in re-breathing methods of determining pulmonary capillary blood flow and cardiac output.
The carbon dioxide Fick equation: EQU Q=VCO.sub.2 /(CvCO.sub.2 -CaCO.sub.2),
where Q is cardiac output, CvCO.sub.2 is carbon dioxide content of the venous blood of the patient, and CaCO.sub.2 is the carbon dioxide content of the arterial blood of the patient, has been employed to non-invasively determine the cardiac output of a patient. The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal, or the fraction of respiratory gases that comprises carbon dioxide or "carbon dioxide fraction", times the rate of flow over an entire breath. The volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or intrapulmonary shunt.
The partial pressure of end tidal carbon dioxide (PetCO.sub.2 or etCO.sub.2) is also measured in re-breathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO.sub.2) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO.sub.2).
Re-breathing is typically employed either to non-invasively estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing). Re-breathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During re-breathing, the carbon dioxide elimination of the patient is less than during normal breathing. Re-breathing during which the carbon dioxide elimination decreases to near zero is typically referred to as total re-breathing. Re-breathing that causes some decrease, but not a total cessation of carbon dioxide elimination, is typically referred to as partial re-breathing.
Re-breathing is typically conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates an exemplary re-breathing circuit 50 that includes a tubular airway 52 that communicates air flow to and from the lungs of a patient. Tubular airway 52 may be placed in communication with the trachea of the patient by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. A flow meter 72, which is typically referred to as a pneumotachometer, and a carbon dioxide sensor 74, which is typically referred to as a capnometer, are disposed between tubular airway 52 and a length of hose 60, and are exposed to any air that flows through re-breathing circuit 50. Both ends of another length of hose, which is referred to as deadspace 70, communicate with hose 60. The two ends of deadspace 70 are separated from one another by a two-way valve 68, which may be positioned to direct the flow of air through deadspace 70. Deadspace 70 may also include an expandable section 62. A Y-piece 58, disposed on hose 60 opposite flow meter 72 and carbon dioxide sensor 74, facilitates the connection of an inspiratory hose 54 and an expiratory hose 56 to re-breathing circuit 50 and the flow communication of the inspiratory hose 54 and expiratory hose 56 with hose 60. During inhalation, gas flows into inspiratory hose 54 from the atmosphere or a ventilator (not shown). During normal breathing, valve 68 is positioned to prevent inhaled and exhaled air from flowing through deadspace 70. During re-breathing, valve 68 is positioned to direct the flow of exhaled and inhaled gases through deadspace 70.
As the amount of carbon dioxide in the re-breathed gas mixture typically approximates the expected carbon dioxide content of the patient's venous blood, the re-breathed air, which is inhaled from deadspace 70 during re-breathing, includes air that has been exhaled by the patient (i.e., carbon dioxide-rich air).
During total re-breathing, substantially all of the gas inhaled by the patient was expired during the previous breath. Thus, during total re-breathing, the partial pressure of end-tidal carbon dioxide (PetCO.sub.2 or etCO.sub.2) is typically assumed to be equal to the partial pressure of carbon dioxide in the arterial (PaCO.sub.2), venous (PvCO.sub.2), or alveolar (PACO.sub.2) blood of the patient. Total re-breathing processes are based on the assumption that neither pulmonary capillary blood flow nor the content of carbon dioxide in the venous blood of the patient (CvCO.sub.2) changes substantially during the re-breathing process. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve. Thus, the carbon dioxide form of the Fick equation, in which CvCO.sub.2 and CaCO.sub.2 are variables, may be employed to determine cardiac output or pulmonary capillary blood flow.
In partial re-breathing, the patient inhales a mixture of "fresh" gases and gases exhaled during the previous breath. Thus, the patient does not inhale as much carbon dioxide as would be inhaled during a total re-breathing process. Conventional partial re-breathing processes typically employ a differential form of the carbon dioxide Fick equation to determine the pulmonary capillary blood flow or cardiac output of the patient, which do not require knowledge of the carbon dioxide content of the mixed venous blood. This differential form of the carbon dioxide Fick equation considers measurements of carbon dioxide elimination, CvCO.sub.2, and the content of carbon dioxide in the alveolar blood of the patient (CACO) during both normal breathing and the re-breathing process as follows: ##EQU1## where VCO.sub.2B and VCO.sub.2D are the carbon dioxide production of the patient before re-breathing and during the re-breathing process, respectively, CvCO.sub.2B and CvCO.sub.2D are the content of CO.sub.2 of the venous blood of the patient before re-breathing and during the re-breathing process, respectively, and CaCO.sub.2B and CaCO.sub.2D are the content of CO.sub.2 in the arterial blood of the patient before re-breathing and during re-breathing, respectively.
Alternative differential Fick methods of measuring pulmonary capillary blood flow or cardiac output have also been employed. Such differential Fick methods typically include a brief change of PetCO.sub.2 and VCO.sub.2 in response to a change in effective ventilation. This brief change can be accomplished by adjusting the respiratory rate, inspiratory and/or expiratory times, or tidal volume. A brief change in effective ventilation may also be effected by adding CO.sub.2, either directly or by re-breathing. An exemplary differential Fick method that has been used, which is disclosed in Gedeon, A. et al. in 18 Med. & Biol. Eng. & Comput. 411-418 (1980), employs a period of increased ventilation followed immediately by a period of decreased ventilation.
The carbon dioxide elimination of a patient is typically measured over the course of a breath by the following, or an equivalent, equation: EQU VCO.sub.2 =.intg..sub.breath V.function..sub.CO2 dt,
where V is the measured respiratory flow and .function..sub.CO2 is the substantially simultaneously detected carbon dioxide signal, or fraction of the respiratory gases that comprises carbon dioxide or "carbon dioxide fraction".
Due to inaccuracies in respiratory flow measurements, accurate, consistent carbon dioxide elimination measurements are, however, somewhat difficult to obtain. Inaccurate flow measurements may be caused by leaks in the breathing circuit, drift of the baseline respiratory flow value, a non-unity respiratory quotient (RQ) (i.e., RQ is not equal to one), or other error-inducing factors, which are collectively referred to as "noise". Moreover, noise may have different and inconsistent effects on inspiratory flow and expiratory flow.
Breathing circuit leaks, such as endotracheal tube cuff leaks, or simply "cuff leaks", are a significant cause of noise when monitoring the respiration of neonates. The component of a flow monitor that senses expiratory flow may be connected directly to an endotracheal tube cuff, where the endotracheal tube may connect to or adjacent a tube with a different inner diameter. As the patient exhales, gas may escape through leaks around the endotracheal tube cuff and may, therefore, not be measured by the expiratory flow sensor. Differences in the internal diameters in the adjacent, connected tubes may also affect the flow of respiratory gases through the tube. Accordingly, the expiratory flow measurement may be inaccurate.
Inspiratory flow measurements are less affected by changes in inlet conditions, flow rate, or the escape of air around the cuff. Moreover, inspired gases are typically drier and include fewer particulates than expired gases. Accordingly, with respect to noise, inspiratory flow measurements are typically more reliable and accurate than expiratory flow measurements. For the same reasons, the volume of gas inhaled by the patient may be calculated with greater reliability and accuracy than the volume of gas exhaled by the patient.
When the preceding equation is employed to calculate the carbon dioxide elimination of the patient from the respiratory flow and carbon dioxide fraction measurements over an entire breath, such noise-induced inaccuracies in either the expiratory flow, the inspiratory flow, or both may cause inaccuracies in the carbon dioxide elimination determination or inconsistencies between carbon dioxide elimination determinations.
Accordingly, there is a need for a method of consistently and accurately calculating carbon dioxide elimination in the presence of noise relative to the consistency and accuracy of conventional methods.